1. The Field of the Invention
The present invention relates to optical devices for measuring light. In particular, the present invention relates to a compact spectrometer device for use in color discrimination or evaluation.
2. The Relevant Technology
Optical devices known generally as spectrometers have been developed for measuring and analyzing the spectral or color content of electromagnetic radiation in the frequency range or spectrum of optical wavelengths. These include from ultraviolet, through visible, to near-infrared wavelengths, which include the portion of the electromagnetic spectrum producing photoelectric effects, referred to herein as xe2x80x9clight.xe2x80x9d Various kinds of opto-electronic devices are used for both imaging applications, such as by inspecting the spectral reflectance characteristics of a two-dimensional object, and for non-imaging applications.
Spectrometric measurements of light are performed in basically two ways, including dispersion-based techniques and filter-based techniques. In the dispersion-based approach, a radiation dispersion device such as a prism or diffraction grating is used to separate the incident polychromatic light into its spectral contents. The spectrally separated light is then projected onto a photodetector to measure the relative intensity in each spectral range. While dispersion-based devices can be effectively used in some applications, they have the disadvantage of being easily knocked out of alignment during use, and are thus not suitable for more rigorous applications in the field.
In the filter-based approach to spectral measurement, various types of optical filters are used in conjunction with photodetectors to measure and analyze light. For example, in one approach, a single band-pass filter is placed over a detector to measure a single spectral band of the incident light. In another variation of the filter-based technique, a filter wheel on which several filters are mounted is used in conjunction with a single photodetector or several photodetectors. Alternatively, the discrete filters in the filter wheel can be replaced with a continuous circular variable filter (CVF), which is placed over a detector. Further, the CVF may be placed over several detectors to provide simultaneous spectra in a limited number of bands. These filter-based techniques are limited for practical reasons to use in low resolution spectral measurements of a few bands of light and to non-contiguous bands only.
Other spectrometer devices have been developed that utilize linear variable filters in an attempt to enhance light measuring capabilities. For example, U.S. Pat. No. 5,166,755 to Gat (hereinafter xe2x80x9cGatxe2x80x9d) discloses a spectrometer apparatus including a spectrum resolving sensor containing an opto-electronic monolithic array of photosensitive elements which form a photodetector, and a continuous variable optical filter such as a linear variable filter (LVF) that is placed in an overlaying relationship with the photodetector. The LVF and photodetector are mounted in a single housing which serves to support at least the filter and the photodetector array in a unitary sensor assembly. The LVF is formed by depositing optical coatings directly onto the photodetector array, or a preformed LVF may be positioned in contact with or slightly above the array.
The device of Gat and other similar spectrometer devices have the disadvantage of containing no optical system disposed between the LVF and the photodetector array to aid in reducing the numerical aperture of the light incident on the array. This is important because the LVF works properly only within a limited cone angle of light (numeric aperture). Outside this limited angle, light of certain characteristic wavelengths may be transmitted through the LVF to the photodetector array at positions where the light would normally be reflected by the spectrally selective bandpass of the LVF. This effect is commonly referred to in the art as xe2x80x9cblue shiftxe2x80x9d of the band-pass filter. The detector array is capable of receiving light within the full hemisphere and will detect these transmitted wavelengths when placed in contact with or very close to the LVF. In addition, if the LVF is spaced apart from the surface of the detector array, light emitted from one position of the LVF may reach more than one element of the detector array, thereby limiting the spectral resolving power of the LVF. In both cases, the net result is reduced resolution and an increased stray light characteristic of the device.
Accordingly, there is a need for an optical/detector device that overcomes or avoids the above problems and limitations.
It is an object of the invention to provide an optical system to aid in reducing the numerical aperture of light incident on a detector array.
It is another object of the invention to provide a sensor assembly for a spectrometer device with a uniform intensity distribution of light at the image plane of the detector array.
It is another object of the invention to provide a sensor assembly for a spectrometer device with an optical coupling mechanism between an optical filter and an optical detector that avoids any loss of resolution.
It is a further object of the invention to provide a smaller, more lightweight spectrometer device.
It is yet a further object of the invention to provide a more rugged and stable compact spectrometer device.
It is another object of the invention to provide a color measuring sensor assembly that can be used in a compact spectrometer device such as a colorimeter having a rugged construction.
It is a further object of the invention to provide a color measuring sensor assembly than can be employed in a variety of applications for use in color discrimination or evaluation.
To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein, a color measuring sensor assembly is provided for use in manufacturing a spectrometer device such as a compact, portable colorimeter. The sensor assembly generally comprises an optical filter for selectively transmitting light, an optical detector for measuring the spectral characteristics of the light transmitted through the filter, and a coherent fiber faceplate interposed between the filter and the detector such that a light beam propagating through the fiber faceplate from the filter to the detector projects an upright, noninverted image of the filter onto a photosensitive surface of the detector. The sensor assembly can also include electronic structure for reading electrical signals from the detector to provide an output representative of the spectral characteristics of the light.
In one embodiment of the invention, the sensor assembly comprises a linear variable filter, a linear detector array having a photosensitive surface positioned directly opposite from the linear variable filter a predetermined distance, and a coherent fiber faceplate including a plurality of optical fibers interposed between the linear variable filter and the linear detector array. A light beam propagating through the optical fibers from the linear variable filter to the linear detector array projects an upright, noninverted image of the linear variable filter onto the photosensitive surface of the linear detector array.
The sensor assembly of the invention can be used to form a compact spectrometer device such as a portable colorimeter by operatively connecting the sensor assembly to various electronic and mechanical components, such as an illumination source, a detector circuit module, a signal processing module, and an output module. Such a spectrometer device can be employed in a variety of applications for color discrimination or spectral evaluation, including color matching or identification of various products, processing controls, medical diagnostics, printer calibration, agricultural applications, combustion monitoring, environmental monitoring, and in other areas requiring color measurement and spectral analysis.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.